Generally, one pixel of an Organic Light-Emitting Diode (OLED) display device comprises sub pixels of three colors of Red (R), Green (G) and Blue (B). Pixels Per Inch (PPI) determines the delicate degree of an image displayed by the display device. The higher the PPI is, the more delicate the image is. The key to increase the PPI is how to reduce the size of a single sub pixel.
At present, pixels of the OLED display device are formed by a vacuum evaporation process with a Fine Metal Mask (FMM). In the forming process, regions where the sub pixels of two colors are to be formed are shielded by the mask whereas a region where the sub pixel of the remaining one color is to be formed is exposed, so as to form the sub pixel of the remaining one color by evaporation. As shown in FIG. 1, an upper part of the diagram illustrates a plane view of a mask, and a lower part of the diagram illustrates a sectional view of the mask; the mask comprises a shielding region 101 and an opening region 102, and a side wall of the shielding region 101 has a certain side wall inclined angle θ. The smaller the side wall inclined angle θ is, the larger an opening on the top of the opening region 102 is, then particles more easily get to an edge at the bottom of the opening region 102, and the size and shape of the evaporated sub pixel are closer to ideal size and shape. That is, the size of the side wall inclined angle θ determines the quality of the evaporation effect. At present, the side wall inclined angle θ is generally 40°-60°.
If d denotes a thickness of the mask, w denotes a width of the shielding region 101, and s denotes a width of the opening region 102, then a width of one sub pixel is p=s=0.5 w. If a is a width of a vertical projection of the side wall of the shielding region 101 on the bottom of the shielding region 101, and b is a width of a vertical projection of the top portion of the shielding region 101 on the bottom of the shielding region 101, then the width of the shielding region 101 is w=2a+b. Therefore, the width of one sub pixel is p=0.5(2a+b)=a+0.5b=(d/tan θ)+0.5 b. It is thus clear that in a case where θ is given, the width of the sub pixel is limited to the thickness d of the mask; the thinner the mask is, the smaller the sub pixels is, and the larger the PPI is.
However, the thinner the mask is, the more difficult the manufacturing process of the mask is, and the higher the manufacturing cost of the mask is. At present, the thickness of the mask used for forming the pixels of the OLED by evaporation is usually 40 μm, and the smallest is 30 μm. However, even the mask with the smallest thickness of 30 μm is adopted, the PPI of the OLED is still not greatly improved, which results in that the delicate degree of the image of the OLED display device cannot be further improved.